Being that Xonotic is a community-driven, open-source project, you do not need our permission to get started creating stuff for use in the game. You could learn to make maps, 3D models, textures, code patches, scripts, sounds or music and those things may be used by people within the community.

If you already have experience, then the first step would be to introduce yourself to the community and give some information about your background and give us an idea of what you want to do.

If you don't have experience, but you have the intention of learning, there are many people within the Xonotic community who will help point you in the right direction. There are also a lot of resources listed in the Wiki.

As you begin to produce stuff, share it with us on the forum. It is also highly suggested to interact with on the IRC channel, #xonotic on irc.quakenet.org